Bug in staking contract

[ethanfrey]

With the Addr refactor in v0.14.0-beta2, we broke the staking contract.

When testing this in wasmd upgrade, I get:

Error Trace:    staking_test.go:208
                                                        staking_test.go:550
                Error:          Received unexpected error:
                                
                                github.com/CosmWasm/wasmd/x/wasm/keeper.Keeper.instantiate
                                        /home/ethan/go/src/github.com/cosmwasm/wasmd/x/wasm/keeper/keeper.go:273
                                github.com/CosmWasm/wasmd/x/wasm/keeper.PermissionedKeeper.Instantiate
                                        /home/ethan/go/src/github.com/cosmwasm/wasmd/x/wasm/keeper/contract_keeper.go:43
                                github.com/CosmWasm/wasmd/x/wasm/keeper.initializeStaking
                                        /home/ethan/go/src/github.com/cosmwasm/wasmd/x/wasm/keeper/staking_test.go:207
                                github.com/CosmWasm/wasmd/x/wasm/keeper.TestQueryStakingPlugin
                                        /home/ethan/go/src/github.com/cosmwasm/wasmd/x/wasm/keeper/staking_test.go:550
                                Generic error: addr_validate errored: invalid Bech32 prefix; expected cosmos, got cosmosvaloper: instantiate wasm contract failed

The validator (valoper) addresses must always be unverified strings as we cannot validate them.

We had this issue in wasmd some time ago: CosmWasm/wasmd#264 and concluded that we cannot validate both AccAddress and ValOperAddress. Unless you want to extend the API (which I found unnecessary), let's keep validators as string. You can verify them in init simply by making a query using them.